Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of miniature devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Charged particle beams include ion beams and electron beams.
Ions in a focused beam typically have sufficient momentum to micromachine by physically ejecting material from a surface. Because electrons are much lighter than ions, electron beams are typically limited to removing material by inducing a chemical reaction between an etchant vapor and the substrate. Both ion beams and electron beams can be used to image a surface at a greater magnification and higher resolution than can be achieved by the best optical microscopes.
Since ion beams tend to damage sample surfaces even when used to image, ion beam columns are often combined with electron beam columns in dual beam systems. Such systems often include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and an ion beam system, such as a focused or shaped beam system, that can be used to alter workpieces and to form images. Dual beam systems including a liquid metal focused ion beam and an electron beam are well known.
Focused ion beam milling in many instances are unacceptably slow for some micromachining applications. Other techniques, such as milling with a femtosecond laser can be used for faster material removal but the resolution of these techniques is lower than a typical LMIS FIB system. Lasers are typically capable of supplying energy to a substrate at a much higher rate than charged particle beams, and so lasers typically have much higher material removal rates (typically up to 7×106 μm3/s for a 1 kHz laser pulse repetition rate) than charged particle beams (typically 0.1 to 3.0 μm3/s for a Gallium FIB). Laser systems use several different mechanisms for micromachining, including laser ablation, in which energy supplied rapidly to a small volume causes atoms to be explosively expelled from the substrate. All such methods for rapid removal of material from a substrate using a laser beam will be collectively referred to herein as laser beam milling.
The combination of a charged particle beam system with a laser beam system can demonstrate the advantages of both. For example, combining a high resolution LMIS FIB with a femtosecond laser allows the laser beam to be used for rapid material removal and the ion beam to be used for high precision micromachining in order to provide an extended range of milling applications within the same system. The combination of an electron beam system, either alone or in conjunction with a FIB, allows for nondestructive imaging of a sample.
FIG. 1 shows a prior art dual beam system 100 having a combination charged particle beam column 101 and laser 104. Such a dual beam system is described in U.S. Pat. App. No. 2011/0248164 by Marcus Straw et al., for “Combination Laser and Charged Particle Beam System,” which is assigned to the assignee of the present application, and which is hereby incorporated by reference. U.S. Pat. App. No. 2011/0248164 is not admitted to be prior art by its inclusion in this Background section. As shown in the schematic drawing of FIG. 1, the laser beam 102 from laser 104 is focused by lens 106 located inside the vacuum chamber 108 into a converging laser beam 120. The laser beam 102 enters the chamber through a window 110. A single lens 106 or group of lenses (not shown) located adjacent to the charged particle beam 112 is used to focus the laser beam 120 such that it is either coincident and confocal with, or adjacent to, the charged particle beam 112 (produced by charged particle beam focusing column 101) as it impacts the sample 114 at location 116.
Integrating a laser beam system with a charged particle beam system provides significant challenges. Problems may arise in spatially stabilizing the laser beam that is used in conjunction with a charged particle beam. The stability of the laser is determined by its ability to precisely maintain its direction as well as its initial position with the output aperture. The laser beam position may drift, however, over time with variations in temperature, mechanical vibrations inside the laser, and other environmental conditions. Periodic re-alignment of the laser beam is therefore required to compensate for the drift. Aligning a laser beam within a charged particle beam system is currently a very tedious and time consuming manual process and requires significant expertise. Automated beam positioning in laser beam systems is well known. See “Automatic beam alignment system for a pulsed infrared laser”, Review of Scientific Instruments 80, 013102 (2009). Past systems usually use a controller that receives signals from beam position detectors, and consequently issue commands for motorized optical elements (e.g., adjustable mirrors) in order to maintain proper alignment of the beam.
Unfortunately, other than aligning the laser beam manually, there is currently no practical system that allows for the convenient alignment of the laser beam positioning in charge particle beam systems. The small sample chamber of a charged particle beam system makes it difficult to house components needed for beam alignment systems. What is needed is a method and apparatus for a convenient way to align a laser beam within a charged particle beam system without the need for performing the alignment manually.